1. Field of the Invention
The present invention relates to sector beam antennas. More particularly, the present invention relates to a method for designing a sector beam antenna with a high gain-area-product (GAP).
While the invention is described herein with respect to a particular implementation with reference to an illustrative embodiment, it is understood that the invention is not limited thereto. Those of ordinary skill in the art will recognize additional applications of the teachings provided herein within the scope of the present invention.
2. Description of the Related Art
Where it is necessary to provide area coverage by an antenn, i.e., for communication satellites, it is often desirable to provide the highest possible gain with uniform coverage. In a communication satellite, for example, it may be desirable to provide uniform coverage within a designated area such as the continental United States. Area coverage is currently accomplished using antennas constructed in accordance with conventional design techniques. In some cases, several antennas are used to provide overlapping sector beams. This approach may be somewhat elaborate and require the coordination of a cluster of multiple geosynchronous satellites, i.e., one for each section of the regional area of coverage. See U.S. Pat. No. 4,375,697 to Visher.
Another common solution is to provide a single antenna system with a multiple feed array shaped roughly in proportion to the region intended to be covered. The electromagnetic signal energy is apportioned among the feed elements. The reflector projects a set of overlapping beams in order to attempt to achieve full coverage of the regional area with approximately the same gain factor over the entire area. These systems are typically complex, using computerized assistance to select the optimum arrangement of amplitudes and phases needed to coordinate the excitations. Also, such systems often have high power requirements which may be difficult to achieve in a particular application.
Although it is well known that a useful figure of merit for sector beam antennas is the gain-area-product (GAP), conventional sector beam antennas are designed to maximize the peak gain. (The GAP is the product of the minimum gain of the antenna, in the coverage area, and the angular coverage area of the coverage region.) The Antenna Engineering Handbook by H. Jasik 1961 (page 2-14) gives a relationship between gain and beamwidth which results in a theoretical GAP value of 10,600 deg.sup.2 for antennas of traditional design. This agrees with current antenna practice, which achieves coverage beams with GAP values ranging from 10,000 to 15,000 deg.sup.2. For a theoretically ideal sector beam with uniform gain within the coverage region and with no gain outside the coverage region, the GAP is 41,253 deg.sup.2. Thus, current practice produces antenna beams with GAP values of 25% to 35% of the maximum achievable gain-area-product.